/Engine/Source/Runtime/InteractiveToolsFramework/Public/InteractiveTool.h
If PropType isCInequalityComparable, allow the two-argument constructor and default to using the type's existing != operator. If PropType is notCInequalityComparable, the caller must use the three-argument constructor to provide a "not equal" function.
template<typename Q, typename> TPropertyWatcher &40; const PropType & Property, FChangedCallback OnChangedIn &41;